The White House on Wednesday invoked the legal principle of executive privilege to block the release of the unredacted Mueller report on Russian election meddling, challenging U.S. House Democrats as they met to vote on holding Attorney General William Barr in contempt for refusing to hand over the document.

The White House’s move escalated a constitutional clash between the Democratic-controlled House of Representatives and Republican President Donald Trump over its powers to investigate him, his administration, his family and his business interests.

House Speaker Nancy Pelosi, the top Democrat in Congress, said Trump’s moves to thwart subpoenas were obstructing oversight by lawmakers and inquiries into Russian interference to boost Trump’s candidacy in the 2016 U.S. election, the subject of Special Counsel Robert Mueller’s report.

“Every single day the president is making the case. He’s becoming self-impeachable,” Pelosi told the Washington Post, referring to the impeachment process in Congress to remove a president from office. Pelosi added that Barr, the top U.S. law enforcement official and a Trump appointee, should be held in contempt of Congress.

Barr last month released a redacted, 448-page version of Mueller’s report on the findings of his 22-month inquiry. Barr has refused to comply with a subpoena by the House Judiciary Committee for an unredacted version and its underlying evidence.

Executive privilege is sometimes claimed by U.S. presidents to keep other branches of government from getting access to information about internal executive branch information.